It is conventional practice in the oil and gas industry to cement casing in well bores. The cement is placed in the annulus between the casing and the well bore wall, and is intended to secure and support the casing in the well bore and to isolate various formations from one another by preventing migration of formation fluids up and down the well bore. The cement is generally pumped down the casing interior, out into the well bore annulus, and back up toward the surface to the desired level. There are numerous techniques and apparatus employed in "cementing," as it is termed, one of the most common being to place a cement basket at or near the lowest end of the casing, and to introduce cement into the well bore annulus immediately above the basket, the basket keeping the cement from travelling below the end of the casing to a lower formation, a particularly undesirable result if the lower formation is a low pressure or soft formation, easily damaged by the hydrostatic pressure of the cement being pumped into the annulus.
Prior art cement baskets and devices incorporating such baskets lack the capability to automatically fill the casing as it is lowered into the well bore, thus causing excessive "float" of the casing as it is supported by the well bore fluid, usually drilling mud. Furthermore, the prior art cement baskets in general use require that an additional prior art device, the cementer or cementing collar, be placed above the basket in the casing string in order to place the cement outside the casing. Another prior art device, a separate "float collar" or "float shoe" is required if the operator desires to control the fill rate of the casing as it is floated into the well bore. In addition, the cement baskets of the prior art devices are retained in a collapsed mode as the casing is run into the well by a tie band around the top of the basket, which band is supposed to release and permit the basket to expand when the cementing operation is commenced. However, at cementing rates below six to eight barrels per minute, the tie band often fails to completely release, thus preventing the basket from opening freely and allowing the cement to travel downward to the formation below the basket, damaging that formation as well as rendering the casing cement job incomplete.